This invention relates to fibre reinforced plastic structures and improved methods and apparatus for producing same.
Fibre reinforced plastic materials are currently finding wide application in the production of various structures such as panels, hoods, doors, tanks, boats and pipes which require such features as corrosion resistance and a high strength to weight ratio. Such structures are generally formed by placing resin coated high tensile strength filaments or fibres on suitable moulds or mandrels, and subsequently curing the resin and removing the fibre reinforced plastic part.
Various methods for providing reinforcement in desired regions of the structure are employed including using woven high strength fabric sheets, laying high strength filaments either as individual filaments or as sheets of filaments in the desired plane of high strength. The method of laying filaments as individual filaments in the desired plane of high strength involves taking continuous strand fiberglass, cutting it to a length and laying it by hand along the areas requiring high strength glass. This can be done from continuous strand glass which consists of gathered filaments wound into a cylindrical package. Another form of this continuous strand material is where many sets of gathered filaments are held together with small amounts of cross-wise stitching and this gathered multiple strand material is cut to length and laid in the described plane of high strength by hand to provide a reinforcement in the desired regions of the structure. Other methods include the incorporation of chopped, randomly oriented fibres into the laminate construction, and helically winding and/or hoop winding filaments, tapes or mats to form cylindrical wall structures. Woven, high strength fabric sheets are also commonly used as reinforcement.
Woven fabric sheet reinforcement is quite expensive as it can be difficult to apply to complex shapes and can also be quite wasteful in that it provides two directional lines of strength even where uni-directional strength might only be needed. The use of high strength filaments either as individual filaments or sheets of filaments produces relatively good products but again fabrication tends to be complex and expensive. The use of random oriented chopped rovings is generally economical in respect to labour requirements but can be both costly and inefficient in those areas where high strength is needed in certain planes of stress as excessive random fibre deposition is required in order to obtain sufficient amounts of high strength filaments in those planes.
With particular reference to the manufacture of hollow bodies such as pipe, hollow poles etc., it is noted that the majority of fibre reinforced plastic piping made at the present time is produced by a filament winding process in which continuous glass fibres are impregnated with resin and applied in a helical pattern on a rotating mandrel. The fibres are deposited by a carriage which moves back and forth along the length of the mandrel. A pipe made by this process has excellent resistance to internal pressure but suffers from severe structural and economic drawbacks. For example, as a result of the helical structure of the glass reinforcement, the longitudinal strength of the pipe is low compared with the strength in other directions. Typically, the longitudinal strength in a bending test is in the order of only 6000 pounds per square inch while the ultimate hoop strength under hydrostatic pressures is in the order of 60,000 pounds per square inch,--a ten fold difference. When reinforced plastics pipe of this nature is supported from hangers, these have to be spaced much closer together than is needed with metal pipes which imposes a heavy cost factor and often a severe engineering inconvenience. In addition to this, at the two ends of the mandrel, where the direction of the lay of glass fibres is reversed, the helical pattern is distorted and an irregular build-up of material occurs which has to be discarded. The loss of material thus incurred varies between 5% and 20% depending on the diameter and length of pipe and on the angle of winding.
It is evident that the first drawback noted above, i.e. longitudinal weakness, can be substantially overcome by incorporating longitudinal reinforcement in the pipe wall. Three principal processes are used at present to achieve this. The first process, of an intermittent nature, involves a carriage which travels back and forth along a mandrel whose rotation is temporarily stopped and which deposits glass fibres in the longitudinal direction. After each longitudinal layer is deposited, a layer is applied in the hoop direction with the mandrel in rotation. This process has the same drawbacks as the helical winding system in that it involves substantial end losses, handling losses, involves costly large mandrels and it is furthermore mechanically complex and provides only a relatively low production rate. A still further process involves the use of a paper core which is continuously produced and then covered by alternating layers of continuous longitudinal fibres and hoop wound fibres. Its main drawbacks are the necessity of building a paper core of adequate strength which subsequently has to be removed and discarded together with its limitation to relatively small diameters (about 12" maximum) because of the enormous number of longitudinal fibres required in large diameter pipes. A still further and somewhat more successful process provides longitudinal reinforcement by providing a carrier type having transversely oriented parallel cut fibres secured thereto. The carrier tape is then wrapped in a near circumferential spiral on a revolving cylindrical mandrel with the turns of tape being in partially overlapping relation thereby to provide the desired longitudinal reinforcement. One of the principal drawbacks of this process is the need for a carrier tape, the production of which involves a separate manufacturing step.